1. Field of the Invention
This invention relates to a radiation imaging panel adapted for use in a radiation imaging apparatus, such as an X-ray imaging apparatus. This invention particularly relates to a photo-conductor, which may be utilized for constituting the radiation imaging panel.
2. Description of the Related Art
There have heretofore been proposed X-ray imaging panels designed for use in a medical X-ray image recording operation, such that a radiation dose delivered to an object during the medical X-ray image recording operation may be kept small, and such that the image quality of an image and its capability of serving as an effective tool in, particularly, the efficient and accurate diagnosis of an illness may be enhanced. With the proposed X-ray imaging panels, a photo-conductor sensitive to X-rays is employed as a photosensitive material. The photo-conductor is exposed to X-rays carrying X-ray image information, and an electrostatic latent image is thereby formed on the photo-conductor. Thereafter, the electrostatic latent image, which has been formed on the photo-conductor, is read out by use of light or a plurality of electrodes. The techniques utilizing the X-ray imaging panels have advantages over the known photo-fluorography utilizing TV image pickup tubes in that an image is capable of being obtained with a high resolution.
Specifically, when X-rays are irradiated to a charge forming layer located in the X-ray imaging panel, electric charges corresponding to X-ray energy are formed in the charge forming layer. The thus formed electric charges are read out as an electric signal. The photo-conductor described above acts as the charge forming layer. As the material for the photo-conductor, amorphous selenium has heretofore been used. However, ordinarily, amorphous selenium has the problems in that it is necessary for the layer thickness of the photo-conductor to be set to be large (e.g., at least 500 μm) because of a low X-ray absorptivity.
However, if the layer thickness of the photo-conductor is set to be large, the problems will occur in that the speed, with which the electrostatic latent image is read out, becomes low. Also, the problems will occur in that, since a high voltage is applied across the photo-conductor at least during a period from the beginning of the read-out operation after the formation of the electrostatic latent image to the end of the read-out operation, a dark current becomes large, electric charges occurring due to the dark current are added to the latent image charges, and the contrast in a low dose region becomes low. Further, since the high voltage is applied across the photo-conductor, device deterioration is apt to occur, durability becomes low, and electric noise is apt to occur. Furthermore, ordinarily, the photo-conductor is formed with a vacuum evaporation technique. Therefore, considerable time is required to grow the photo-conductor up to the large layer thickness as described above with the vacuum evaporation technique, and management of the growth of the photo-conductor is not easy to perform. As a result, the production cost of the photo-conductor is not capable of being kept low, and the cost of the X-ray imaging panel is not capable of being kept low.
Also, amorphous selenium has toxicity and exhibits a glass transition temperature of approximately 43° C. At temperatures higher than approximately 43° C., amorphous selenium is set in a metastable state, in which crystallization proceeds, and therefore a marked alteration of the characteristics occurs with the passage of time. Accordingly, amorphous selenium has the problems in that particular management is necessary at the time of use and at the time of storage.
Because of the problems described above, it has been studied to utilize materials for the photo-conductor other than amorphous selenium. By way of example, as a substance for constituting the photo-conductor, there has been proposed a bismuth oxide type of composite oxide (sillenite). The proposed bismuth oxide type of the composite oxide may be represented by the formula BixMOy, in which M represents at least one kind of element selected from the group consisting of Ge, Si, and Ti, x represents a number satisfying the condition 10≦x≦14, and y represents the stoichiometric oxygen atom number in accordance with M and x. The proposed bismuth oxide type of the composite oxide is described in, for example, each of Japanese Unexamined Patent Publication Nos. 11 (1999)-237478 and 2000-249769. Also, a photo-conductor containing single crystal Bi12SiO20 is described in, for example, “Transport processes of photoinduced carriers in Bi12SiO20”, S. L. Hou et al., J. Appl. Phys., Vol. 44, No. 6, pp. 2652-2658, 1973. Further, single crystal Bi12GeO20 and single crystal Bi12SiO20 are described in, for example, “Properties of Pure and Doped Bi12GeO20 and Bi12SiO20Crystals”, B. C. Grabmaier and R. Oberschmid, Phys. Stat. Sol. (a) 96, pp. 199-210, 1986. With each of the proposed bismuth oxide types of the composite oxides, it is expected that the efficiency, with which the X-rays are converted into the electric charges, will be capable of being enhanced.
Furthermore, in, for example, U.S. Patent Application Publication No. 20050214581, there has been proposed a radiation imaging panel using a particle coating film or a sintered film utilizing Bi12MO20, in which M represents at least one kind of element selected from the group consisting of Ge, Si, and Ti. It is described therein that, in cases where the photo-conductor takes on the form of a polycrystal, the efficiency with which electric charges having been generated are collected is capable of being enhanced, electric noise is capable of being kept low, and the sensitivity is capable of being enhanced.
As for image formability of the radiation imaging panel, it is necessary for the sensitivity to be enhanced through the enhancement of the electric charge collecting efficiency as described above, and it is markedly important to obtain image stability at the time of iterated image recording and read-out operations. Particularly, in the cases of X-ray images for medical diagnosis, if a ghost or an alteration of gray level gradation contrast arises during the iterated image recording and read-out operations, an erroneous diagnosis will be caused to occur. Therefore, it is important for the ghost and the alteration of gray level gradation contrast to be suppressed.
The image ghost and the alteration of contrast are caused to occur by a decrease of intensity of a signal obtained from a radiation detecting element, which decrease arises due to iterated irradiation of radiation, or by the occurrence of a residual signal after the irradiation of the radiation. As for the photo-conductor constituting the radiation detecting element, the foregoing means that an electric current generated by the irradiation of the radiation alters, and that the electric current at the time other than the irradiation of the radiation alters.